This blog shows you two different types of finderscopes. It describes what a finderscope is, how it’s useful, how it attaches to your main scope. Not all new astronomers know that they have to align the finderscope with the main scope. So most importantly, the blog shows you how to adjust your finderscope to help you get your main scope pointing where you want it quickly.
What is a finderscope and how to use it
Have you ever tried to point your high magnification telescope at a target that you can see with your eyes, like Venus? You roughly point the telescope, and then peer through the eyepiece and see – nothing. So you use the fine controls on the mount and hunt around … and around … . After about ten minutes you look away from the eyepiece and discover that you’re not even pointing anywhere near the target any more, and you mutter a few words that would have you banned from the pub. You can see it right there! How can you not see it in your telescope?!
You can save all this swearing if you use a finderscope. This little gizmo shows a low-magnification view of what you’re pointed at, a bit like a gun sight. After you roughly line your main scope up on the target, you look through the finderscope and see what you’re searching for. Then you can adjust the mount a second time, and then you look through the scope and voila! you can see your target.
Different types of finderscope
The first common type is the optical finderscope, which is really just a small, low-powered refractor telescope that is attached to the side of your main scope. The second type is the “red-dot”, which is actually a heads-up display, much like a lot of cars have these days. The one thing that you have to remember about the red-dot is that you hold your eye well back from it. The red-dot has zero magnification so you can do this. It takes a bit of getting used to.
The photo below shows both of these types of finderscope, with the optical finderscope on the left, and the red-dot on the right.
There are other types of finderscopes, most commonly the Telrad, but this is quite similar to the red-dot, so I won’t go into detail. Personally, I’m not a fan of the Telrad – it’s a bit large for my liking, but that’s purely a matter of taste.
How to attach them
Most scopes have a place for a finderscope. The saxon 1206 that I’ve been playing with lately certainly does. It’s the box-shaped fitting with the thumb-screw.
Don’t worry if your scope doesn’t have a place for a finderscope. You can get different types of mounts for one. Your scope will probably have holes drilled in a convenient place so you can attach one, like these ones. They’re not expensive.
Before they’re useful, you have to adjust all finderscopes so that they’re pointing in the same direction as the main scope. This is best done during the day, lining them up with some distant but stationary object. Obviously, take care not to bump them after you’ve done all this good work!
The procedure is pretty much the same across both finderscopes. You aim the main scope on your target, then adjust the finderscope to the same target.
I decided to adjust the red-dot on the saxon 1206 I was reviewing. I took it upstairs at my house where I can get a decent view of some freeway lights. Through the main scope, this is what the light looks like. I got the light itself in the middle of the field, and took this photo using my mobile phone and a saxon ScopePix.
Then I turned on the red-dot, sat back and looked into the sight. This is what I saw. Clearly, the red-dot wasn’t pointing at the same place.
Make the adjustment on the red-dot using two knobs – one for up-down and one for left-right. In this photo, the up-down adjustment is on the bottom at the back of the red-dot and the left-right is on the side at the front. The on-off-brightness control for the red-dot is the knob in the middle. Don’t forget to turn it off after you’ve used it or you’ll find you’ve run out of battery next time you need it. (That’s the voice of experience.)
The adjustment itself is pretty straightforward. It’ll only take you a few minutes.
Finally, when you have the finderscope aimed correctly, you can use it to find your target quickly. Take care not to bump the finderscope or you’ll have to aim it all over again, and in darkness that’s not easy. If you’re careful and confident in the mounting point, you can (carefully) remove the finderscope from the main scope and put it back when you need it.
Occasionally, particularly with old equipment, I’ve found that the finderscope won’t adjust sufficiently to be parallel with the main scope. In these cases you might have to use some shim or packing to help you align them.
Next, I removed the red-dot and put on my Orion 50mm Guide scope. It has the same fitting as the red-dot, so it attached to the saxon 1206 in seconds.
Orion actually built the mini-guide scope for autoguiding. However, with a standard eyepiece and a small extension tube it works very well as an optical finderscope. With my 20mm eyepiece, it has a magnification of 8.25 times.
Most optical finderscopes have a crosshair that helps you aim the scope. I’ve attached a photo I took through one a little while back. It’s from a saxon F767AZ Newtonian Reflector. When you’re using crosshairs, you rotate the finderscope so that moving the main scope along one axis moves it along one of the crosshairs. It makes things easier that way. This one was on an alt-azimuth mount, so the crosshairs are vertical and horizontal. I also didn’t use the saxon ScopePix to take the photo, so it’s a bit dodgy. Sorry.
The procedure for adjusting an optical finderscope is very similar to the one for the red-dot. Get the main scope pointed at something in the distance, lock the mount so it doesn’t move, then adjust the finderscope so it’s pointed at the same object.
On my Orion finderscope, the three black adjustment screws work together to aim the finderscope body. Having to think in three directions takes a bit of getting used to, though. Remember to back one screw off while tightening the other or you’ll end up scratching the finderscope or bending the screws.
A finderscope can be very useful, saving you time and frustration. Finderscopes come in different types, but they work in similar ways. They give you a low magnification view of what you main scope is pointing at. You need to align them correctly beforehand, preferably during the day.
Astronomy Hacks by Barbara Fritchman Thompson and Robert Bruce Thompson has an article called Align your Finder, which goes through a similar procedure.
YouTube has a couple of videos on this as well, such as this one. They’re a bit long but probably worth watching once.
Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, how to take photos of the sky, or even how to see some rare birds.